v0.9.17 — Skip empty-track GPX before EFB upload

Highlights

Stops EFB's silent "konnte nicht interpretiert werden, XML-Fehler!" rejection class — the dominant ongoing error category, affecting 11 distinct users since 2026-04-26.

Root cause

Garmin returns a structurally-valid 658-byte GPX shell with an empty <trkseg/> for activities recorded without GPS (indoors, GPS disabled, manual entry). EFB's parser refuses to interpret a track with zero points and surfaces it as a misleading XML parse error. Diagnosed against prod using v0.9.16's debug-upload?include_gpx=1 — verified the same 658-byte stub across two failing users.

Changes

• New internal/garmin.HasTrackPoints(gpxData) — cheap byte-level scan for the <trkpt marker.
• Sync engine skips empty-track GPX before EFB upload, records the activity as upload_status = "no_track_points", counts as skipped (not failed), no retries.
• Activity-loop status switch handles "no_track_points" so subsequent sync runs stay idempotent.

Notes

• No schema migration. synced_activities.upload_status is unconstrained TEXT.
• Backlog cleanup deferred. The 14 existing permanent_failure rows are inert (already skipped by the activity loop). Optionally reclassify via SQL: UPDATE synced_activities SET upload_status='no_track_points' WHERE upload_status='permanent_failure' AND error_message LIKE '%XML-Fehler%';

PR: #41